Add activeTime prop for synced crosshairs

[lndgalante]

Summary

Adds an activeTime prop that renders a crosshair at a given timestamp on any chart — the chart interpolates the Y value(s) internally. Designed for syncing crosshairs across multiple charts via onHover + activeTime.

• Works live or paused, single-series or multi-series
• Interpolates Y from chart data — caller only provides a timestamp
• User hover takes precedence over activeTime
• Includes interactive synced-charts demo (3 charts with shared crosshair)

Usage

const [syncTime, setSyncTime] = useState<number | null>(null)

<Liveline data={dataA} onHover={p => setSyncTime(p?.time ?? null)} activeTime={syncTime} />
<Liveline data={dataB} onHover={p => setSyncTime(p?.time ?? null)} activeTime={syncTime} />

Changed files

File	Change
src/types.ts	Add activeTime to LivelineProps
src/Liveline.tsx	Thread prop through to engine
src/useLivelineEngine.ts	Interpolate Y at given time, compute screen position
src/draw/index.ts	Draw programmatic crosshair, suppress hover when active
dev/main.tsx	Synced charts demo with onHover + activeTime

Demo

CleanShot.2026-03-25.at.00.54.21.mp4

[lndgalante]

cc @benjitaylor amazing lib! Let me know if this implementation looks good to you, needs to be think differently, or needs some improvements.